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April 27, 2012

Earth’s Magnetic Field Gives Pigeons Built-in GPS

by editor

Certain neurons in the brains of pigeons encode the direction and intensity of the Earth´s magnetic field, giving the birds an inborn internal global positioning system, according to a new study published Thursday in the journal Science.

Scientists have long known internal magnetic field receptors exist in many animals, perhaps including humans. However, the current study is the first to actually describe the brain wiring that uses these receptors to provide a sense of direction.

The researchers did it by putting some pigeons in the dark and monitoring their brains.

“We have found cells in the (pigeon) brain that signal the direction, intensity and polarity of an applied magnetic field,” said co-author J. David Dickman, a Baylor College of Medicine neuroscientist, during an interview with Discovery News.

“These three qualities can be used by the brain to compute heading information, like a compass, and latitude on the Earth surface,” he said.

“It is possible that magnetic intensity could also be used to give the bird longitude learned associations of differing regional variations along the Earth surface,” he added.

“Together, these cells could form the basis of determining heading direction and position according to a brain representation of a magnetic Earth surface map.”

Dickman and fellow researcher Le-Qing Wu, also of Baylor College of Medicine, used previous research identifying magnetic receptors in bird beaks, and set out to find the neural network responsible for making sense of those signals.

The researchers placed 7 awake pigeons in a pitch-black room, and used a 3D Tesla coil system to negate the Earth´s natural geomagnetic field and induce an artificial one inside the room.

They used a dark room to cancel out any light-polarization effects, which have been suggested as a possible mechanism for animals´ magnetic navigation capabilities. They also stabilized the birds´ heads so they couldn´t rely on inner-ear cues to determine their directions.

The researchers then adjusted the elevation angles and magnitude of the artificial magnetic field, while simultaneously using gene markers to identify when neurons were activated, focusing on areas that were previously identified as good candidates for processing magnetic signals.

Ultimately, they pinpointed 53 neurons in the birds´ brain stems that showed greatly enhanced activity. Moreover, those cells were most sensitive to the magnetic field ranges that correspond to Earth´s real ones – 20 micro-Tesla (Î¼T) at the magnetic equator to more than 60 Î¼T at the magnetic poles.

Since the Earth´s poles shift over time, the researchers note that these neurons must be somewhat adaptable, either through evolution or brain plasticity.

Wu and Dickman said they believe that the magnetic signals are transmitted to the neurons via the inner ear, or perhaps the beak and/or retinal receptors.

“[The neurons] encode a geomagnetic vector that could be used by the neural population to computationally derive the bird´s position and directional heading,” the researchers wrote in a report about the study.

In other words: A neural global positioning system.

Precisely how these cells are used for orientation and navigation is still unclear.

Wu and Dickman said their findings may apply to other birds, as well as bacteria, honeybees, fish, turtles and even a few mammals. Indeed, all of these creatures are documented as being able to sense and use the Earth´s magnetic field.

In the case of pigeons, their innate navigation ability allows them to travel hundreds of miles.

“The ancient Romans used pigeons to carry messages home from their battles,” Dickman said.

However, while GPS is useful, pigeons rely more upon other orientation cues when honing in on a specific location, such as vision and smell, he added.

Meanwhile, humans still have to ask for directions.

“It is not currently believed that humans possess a magnetic sense,” Dickman said.

“However, humans do have a very elaborate spatial mapping system that helps us navigate our daily routes, like finding the kitchen from our bedroom, or the grocery store when driving from home.”